Moving Picture Experts Group (MPEG) and Video Coding Experts Group (VCEG) have developed an improved and excellent video compression technology over existing MPEG-4 Part 2 and H.263 standards. The new standard is called H.264/AVC (Advanced Video Coding) and was released simultaneously as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264.
The H.264/AVC (hereinafter referred to as ‘H.264’) uses a spatial predictive coding method, which is different from conventional video coding international standards such as MPEG-1, MPEG-2, MPEG-4 Part2 Visual and the like. Conventional methods use “intra prediction” for coefficients transformed in Discrete Cosine Transform Domain (or DCT Transform Domain) to seek higher encoding efficiency resulting in degradation of the subjective video quality at low band transmission bit rates. However, H.264 adopts the method of encoding based on a spatial intra prediction in a spatial domain rather than in a transform domain. An encoder that uses a coding method based on the conventional spatial intra predictions predicts current block information from information of the previously encoded and reconstructed previous blocks, encodes only the difference information between the actual block and the predicted block, and transmits the encoded information to a decoder. Then, the encoder may transmit parameters needed for prediction of the block to the decoder, or the encoder and decoder may be synchronized, so that they share the needed parameters for the decoder to predict the block. In terms of the decoder, the block information to be currently decoded is predicted using previously decoded and reconstructed adjacent block information and then added to the difference information transmitted from the encoder, which reconstructs the block to be decoded. Then, again, if the parameters needed for the prediction are transmitted from the decoder, the parameters can be decoded and used for prediction.
The above described intra prediction may be an intra—4×4 prediction, intra—16×16 prediction, intra—8×8 prediction and the like, where the respective intra predictions include a plurality of prediction modes.
Referring to FIG. 1, the intra—4×4 prediction has nine prediction modes which include a vertical mode, horizontal mode, direct current (DC) mode, diagonal down-left mode, diagonal down-right mode, vertical-right mode, horizontal-down mode, vertical-left mode and horizontal-up mode.
In addition, referring to FIG. 2, the intra—16×16 prediction has four prediction modes which include a vertical mode, horizontal mode, DC mode and plane mode. The intra—8×8 prediction also has four modes similar to the intra—16×16 prediction.
An H.264 encoder selects an optimal prediction mode out of a plurality of prediction modes and performs a prediction. Compression efficiency depends on what prediction mode is selected as the optimal prediction mode and used for a block prediction. In order to select the optimal prediction mode, a prediction of the block is performed with respect to every prediction mode before a cost is calculated using a predetermined cost function, and a prediction mode with the lowest cost is finally selected.
Therefore, in order to determine the optimal prediction mode whether it is intra—4×4 prediction or intra—16×16 prediction, the cost should be calculated after trying every prediction mode. This makes an encoding process complex and lowers the compression efficiency.
In addition, in case there exist identical or similar pixels in the adjacent blocks that produce the correspondingly identical or similar predicted values, a selecting method that requires predictions in all the prediction modes results in unnecessary process and lowers the coding efficiency.